Controllable amplifiers are used, for example, in mobile radio technology. There, the output power of a transmitter in systems having linear modulation methods is controlled or regulated using the input power of the power output stage. Linear modulation methods are used, for example, in the UMTS (Universal Mobile Telecommunications System) and GSM/EDGE (Global System for Mobile Communication/Enhanced Data Rate for GSM Evolution) mobile radio systems.
In order to avoid sudden changes in the gain, it is desirable to control the output power in proportion to an analog control voltage rather than in discrete steps. Amplifiers of this type are referred to as voltage controlled gain amplifiers (VGAs). The characteristic variables of a VGA are not only the output power which can be achieved but also undesirable intermodulation, the noise properties and the power consumption, the latter in portable applications, in particular. In order to satisfy these requirements, bipolar circuit technologies have hitherto normally been used to implement VGAs. In this case, the noise of the transmitter is reduced to required values, with a relatively large outlay in terms of costs and chip area, using a surface acoustic wave (SAW) filter.